Pyridonecarboxylic acid and pharmacologically acceptable salts thereof are superior synthetic antibacterial agents. However, since pyridonecarboxylic acid has carboxylic acid and the dihydropyridine skeleton in a molecule, forming a zwitterion, it shows low solubility in water at a physiological pH, i.e., neutral range. This imposes a problem that an aqueous solution containing pyridonecarboxylic acid or a pharmacologically acceptable salt thereof cannot be formulated into a pharmaceutical preparation having a near neutral pH.
Hardly soluble pyridonecarboxylic acid and a pharmacologically acceptable salt thereof have been conventionally solubilized by, for example, adding an inorganic acid to pyridonecarboxylic lactate (JP-A-60-94910), or adding an excess base to pyridonecarboxylic acid (JP-A-61-180771), or adding a metal compound comprising aluminum, magnesium or zinc to pyridonecarboxylic acid or a salt thereof (JP-A-63-188626). The aqueous solutions thus obtained show changes in pH, which is caused by the solubilizer added, and the toxicity of the solubilizer itself which may cause a local irritation and the like. In addition, the absorption into the living body may decrease due to an interaction between the solubilizer and pyridonecarboxylic acid.
Thus, there has not been provided an aqueous solution containing solubilized pyridonecarboxylic acid, which is safe and useful at a physiological pH, i.e., about neutral pH (pH 6-8).